Investigating Disk Performance Problems

Several conditions must exist in order for you to determine that a disk bottleneck exists. These are a sustained rate of disk activity well above your baseline, persistent disk queues longer than two per disk, and the absence of a significant amount of paging. Without this combination of factors, it is unlikely that you have a bottleneck. However, if you suspect a disk-specific performance problem, monitor the following types of counters:

Although not reflected in disk activity, the rate of interrupt generation by your disk hardware can have a systemwide performance impact. Disk I/O can sometimes generate a sufficient number of interrupts to slow the performance of the processor. Although this does not constitute a "disk" bottleneck, it
is
a processor bottleneck caused by the disk system that can slow the responsiveness of the whole computer. For more information about monitoring disk interrupts and reducing their impact on system performance, see
"Analyzing Processor Activity"
in this book.

The following sections describe how you interpret the values of these counters to reveal or rule out a bottleneck.